1. Field of the Invention
The present invention relates to an image-pickup apparatus and a method for driving the same, adapted for reading signal charges accumulated in a plurality of photodetection portions provided on a predetermined plane, through vertical transfer lines and a horizontal transfer line, which extend perpendicular to each other on the plane.
2. Related Background Art
The conventional electronic cameras and video cameras are equipped with an image-pickup apparatus, for example, composed of CCD image sensing devices or MOS image sensing devices. Some conventional examples of the image-pickup apparatus of this type will be described.
FIG. 1 is a drawing to show the structure of a conventional image-pickup apparatus of a progressive scan type (the first conventional example).
In FIG. 1, the image-pickup apparatus of the first conventional example has photodetection portions 51 of photodiodes or the like arrayed two-dimensionally on an image-pickup surface. Vertical transfer lines 52 (vertical CCD groups) are placed each through a transfer gate 53 between vertical strings of these photodetection portions 51 (vertical lines).
For each of the vertical transfer lines 52, 3-phase electrodes xcfx861-xcfx863 for transfer of charge are provided at regular intervals, and a horizontal transfer line 54 (a horizontal CCD group) is arranged along the horizontal direction on the output end side of the vertical transfer lines 52. An output amplifier 55 is connected to the output side of this horizontal transfer line 54.
In the image-pickup apparatus of this structure signal charges according to quantities of light received by the image-pickup surface are accumulated in the respective photodetection portions 51.
When a high voltage is applied to the electrodes xcfx862, xcfx863, potential wells for accumulating the signal charge are established under the electrodes xcfx862, xcfx863. At this time a vertical length L10 of a potential well is approximately two thirds of a vertical width L20 of a horizontal line, as shown in FIG. 1. A horizontal line means one row or two or more rows of photodetection portions 51 arranged along the horizontal direction and a vertical width of a horizontal line means a width along the vertical direction, of the photodetection portions 51 forming the horizontal line.
The signal charges accumulated in the photodetection portions 51 in this state are transferred at one time to the respective potential wells established in the vertical transfer lines 52, through the transfer gate 53.
When 3-phase driving pulses are then applied to the electrodes xcfx861-xcfx863, the signal charges in the potential wells are successively transferred in the vertical direction.
The signal charges of one horizontal line outputted from the group of vertical transfer lines 52 are taken into the horizontal transfer line 54 during a horizontal blanking interval. The signal charges of one horizontal line thus taken in are transferred in the horizontal direction to the outside over one horizontal scan period. The horizontal scan period means a period necessary only for the signal charges transferred from the respective vertical transfer lines into the horizontal transfer line to be successively transferred and be outputted to the outside, and the horizontal blanking interval means a period in which the transfer operation of signal charge is interrupted in the horizontal transfer line.
The outputs from the horizontal transfer line are subjected to the well-known color signal process and xcex3 correction or the like, thus generating image signals of a frame unit.
Next, FIG. 2 is a drawing to show the structure of a conventional image-pickup apparatus of a two-line-mixed interlace scan type (the second conventional example).
In FIG. 2, the image-pickup apparatus of the second conventional example has photodetection portions 61 of photodiodes or the like arrayed two-dimensionally on the image-pickup surface. Vertical transfer lines 62 (vertical CCD groups) are placed each through transfer gate 63 between the vertical strings of these photodetection portions 61.
For each of the vertical transfer lines 62, 4-phase electrodes xcfx861-xcfx864 for transfer of charge are provided at regular intervals and a horizontal transfer line 64 (a horizontal CCD group) is placed along the horizontal direction on the output end side of the vertical transfer lines 62. An output amplifier 66 is connected to the output side of this horizontal transfer line 64.
The image-pickup apparatus of this structure can perform interlace reading.
Described below is a case where an odd field is read.
First, a high voltage is applied to the electrodes xcfx861-3, whereby potential wells for accumulating the signal charge are established under the electrodes xcfx861-3. At this time the vertical length L11 of each potential well is approximately three quarters of the vertical width L21 of a horizontal line (composed of the photodetection portions in two rows), as shown in FIG. 2.
The signal charges accumulated in the photodetection portions 61 in this state are transferred at once to a potential well established in each of the adjacent vertical transfer lines 62, through the transfer gate 63. At this time signal charges accumulated in the photodetection portions 61 in the n-th row (n is an odd number) and in the (n+1)th row are mixed in one potential well, thus generating a signal charge of one horizontal line.
Then 4-phase driving pulses are applied to the electrodes xcfx861-xcfx864, whereby the signal charges in the potential wells are successively transferred in the vertical direction.
The horizontal transfer line 64 takes in the signal charges of one horizontal line outputted from the group of vertical transfer lines 62 during a horizontal blanking interval. The signal charges of one horizontal line thus taken in are transferred in the horizontal direction over one horizontal scan period.
The outputs from the horizontal transfer line are subjected to the well-known color signal process and xcex3 correction or the like, thus generating image signals of an odd field.
For reading an even field, the signal charges accumulated in the photodetection portions 61 in the n-th row (n is an odd number) and in the (nxe2x88x921)th row are mixed in the potential wells established under the electrodes xcfx864, xcfx861, xcfx862, thus generating signal charges of one horizontal line.
Next, FIG. 3 is a drawing for explaining the operation of a conventional image-pickup apparatus of the CSD method (the third conventional example).
In FIG. 3, the image-pickup apparatus of the third conventional example has electrodes xcfx861-xcfx86m arranged at regular intervals on each vertical transfer line 71. On the output end side of the vertical transfer line 71 a horizontal transfer line 74 is placed through storage area 72 and gate portion 73. This storage area 72 is a region for temporarily storing a signal charge and an electrode xcfx86C for establishing a potential well for storage of charge is placed on the storage area 72. The gate portion 73 is a transfer gate for electrically opening or closing the region between the storage area 72 and the horizontal transfer line 74, and a gate electrode xcfx86T is placed on the gate portion 73.
On the other hand, placed on the horizontal transfer line 74 is an electrode xcfx86H for establishing potential wells for transferring signal charges of one horizontal line taken in from the vertical transfer lines 71, in the horizontal direction.
In the image-pickup apparatus of this structure, a high voltage is applied at once to the electrodes xcfx861-xcfx86m on each vertical transfer line 71, thus establishing a potential well over the entire length of the vertical transfer line 71. In this state, a signal charge in each photodetection portion (not illustrated) forming one horizontal line is transferred to the potential well established in the vertical transfer line 71. (t1 in FIG. 3)
Subsequently, the voltage applied to the electrodes xcfx861-xcfx86m is lowered in the descending order from the other end side of the vertical transfer line 71, whereby the signal charge in the potential well is pushed successively toward the storage area 72 (t2-t5 in FIG. 3).
The signal charge stored in the storage area 72 is transferred through the gate portion 73 to the horizontal transfer line 74 during a horizontal blanking interval (t6-t8 in FIG. 3). The signal charges in the horizontal transfer line 74 are transferred in the horizontal direction over one horizontal scan period (t9 in FIG. 3), and then are successively outputted to the outside of the horizontal transfer line 74.
The above operation (t1-t9 in FIG. 3) is repeated while shifting the reading position of horizontal line, whereby image signals of the entire image-pickup surface are read out to the outside.
The inventor has examined the conventional technologies described above and found the following issues.
In recent years there is the demand for decrease in the width of the vertical transfer lines in order to decrease the chip size of image-pickup devices, in order to increase the number of pixels on the image-pickup surface, or in order to increase the aperture rate per pixel. In the image-pickup apparatus of the first and second conventional examples shown in FIG. 1 and FIG. 2, however, the vertical length L10 or L11 of one potential well must be shorter than the vertical width (the width in the vertical direction) L20 or L21, respectively, of one horizontal line. For preventing a signal charge from overflowing out of a potential well, it is necessary to assure a sufficient horizontal width (a width in the horizontal direction) of each vertical transfer line.
On the other hand, the image-pickup apparatus of the third conventional example shown in FIG. 3 has a larger storage capacity of signal charge than necessary, because the potential well is established over the entire length of each vertical transfer line. Therefore, this is a preferred structure for narrowing the horizontal width of vertical transfer line. The image-pickup apparatus of the third embodiment, however, completes within one horizontal period the sequential operation from the time when the signal charge of each photodetection portion forming one horizontal line is transferred to its corresponding vertical transfer line 71 to the time when the signal charge, after having been transferred in each vertical transfer line, is transferred to the horizontal transfer line 74. Therefore, the vertical transfer lines need to be driven at especially high speed.
For example, supposing 254-stage electrodes are prepared as the transfer electrodes composing the electrode string provided on the vertical transfer line, the signal charge must be transferred through 254 stages within one horizontal blanking interval (approximately 0.25 xcexcsec), so that high-speed driving of at least about 4 MHz must be carried out.
In general, the transfer electrodes provided on the vertical transfer line are connected in parallel, corresponding to the number of horizontal pixels. Therefore, a large capacitive impedance appears in each electrode on the vertical transfer line. For high-speed driving of this large capacity, a large driving current is necessary, which will result in increasing the dissipation power of the image-pickup apparatus. Flow of the large driving current will increases heating amounts of the driving circuit, which could pose a problem of increase of dark current noise or the like. Further, there is also a possibility that the waveform of the driving voltage will become weakened in the high-speed driving of large capacity, so as to cause a transfer leak of signal charge.
The present invention has been accomplished to solve the above issues and an object of the present invention is to provide an image-pickup apparatus and a method for driving the apparatus in structure enabling to decrease the horizontal width of each vertical transfer line and enabling to keep the driving speed low in each vertical transfer line.
Specifically, an image-pickup apparatus according to the present invention comprises at least a plurality of photodetection portions arrayed two-dimensionally on an image-pickup surface, vertical transfer lines each extending adjacent to respective vertical lines being strings of photodetection portions arranged along a vertical direction out of the plurality of photodetection portions and extending along the vertical direction, and a horizontal transfer line provided along a plurality of horizontal lines being one row or two or more rows of photodetection portions arranged along a horizontal direction perpendicular to the vertical direction. Provided on each of the above-stated vertical transfer lines is an electrode string consisting of a plurality of transfer electrodes for establishing potential wells for transferring signal charges accumulated in the photodetection portions. The above horizontal transfer line is a region for successively transferring along the horizontal direction the signal charges having been transferred thereto each from the above vertical transfer lines.
Particularly, for achieving the above object, a method for driving the image-pickup apparatus according to the present invention is a method for, in each of the above plurality of vertical transfer lines, establishing a plurality of potential wells having a length not smaller than a vertical width of each horizontal line and for moving the plurality of potential wells along the vertical direction. In each of the vertical transfer lines, photodetection portions to be read are those of objects successively selected according to a reading order of plural horizontal lines, and a signal charge accumulated in a photodetection portion selected is transferred from the photodetection portion to a vacant potential well at timing when the vacant potential well is established in a portion adjacent to the photodetection portion thus selected.
For realizing the above driving method, the image-pickup apparatus according to the present invention further comprises a plurality of gate structures, provided corresponding to the plurality of respective photodetection portions, for transferring signal charges accumulated in the plurality of photodetection portions to potential wells established in the respectively adjacent vertical transfer lines, and a vertical driving control system for controlling the gate structures corresponding to the photodetection portions composing a horizontal line selected out of the plurality of horizontal lines, in each of the plurality of vertical transfer lines the vertical driving control system successively selecting a plurality of electrode groups each including two or more transfer electrodes arranged consecutively from the electrode string and successively applying a predetermined voltage to the plurality of electrode groups selected.
In the image-pickup apparatus and the method for driving it according to the present invention, since the potential wells having the length not smaller than the vertical width of a horizontal line are established in each of the vertical transfer lines as described, the width of the vertical transfer lines can be set narrower.
In the image-pickup apparatus according to the present invention, in each of the vertical transfer lines the above vertical driving control system simultaneously selects from the above electrode string an electrode group for establishing a potential well for transferring a signal charge accumulated in a photodetection portion forming a first horizontal line selected from the plurality of horizontal lines and an electrode group for establishing a potential well for transferring a signal charge accumulated in a photodetection portion forming a second horizontal line different from the first horizontal line. By this structure, a plurality of potential wells each containing a signal charge are established in each of the above vertical transfer lines at predetermined timing. In other words, in each of the above vertical transfer lines, one potential well or two or more potential wells containing signal charges are moved from positions where the signal charges were transferred through the gate structure, to the output end of each vertical transfer line over a plurality of horizontal scan periods, each being a period in which signal charges are transferred in the horizontal transfer line, or over a plurality of horizontal blanking intervals, each being a period in which the transfer operation of signal charge is interrupted in the horizontal transfer line. In each of the vertical transfer lines, a signal charge in a photodetection portion forming a horizontal line to be read next is transferred to a potential well established in the vertical transfer line, during a period in which a signal charge in a photodetection portion forming a horizontal line selected out of the horizontal lines is transferred in the vertical direction.
As described above, the image-pickup apparatus is not constructed in such arrangement that a signal charge taken out of a photodetection portion forming a horizontal line of a read object is transferred to the output end of each vertical transfer line during one horizontal period (including a horizontal scan period and a horizontal blanking interval), but is constructed in such arrangement that while signal charges accumulated in the photodetection portions in horizontal lines of read objects are successively transferred to each vertical transfer line, the potential wells in which the signal charges are present are gradually transferred toward the output end of each vertical transfer line, thereby permitting efficient transfer of signal charge with keeping low driving speed of the vertical transfer lines.
For realizing the driving method, the above vertical driving control system comprises a first shift register having output ports the number of which is not less than the number of photodetection portions forming a vertical line out of the plural photodetection portions, the first shift register being adapted for selecting gate structures corresponding to respective photodetection portions forming a horizontal line selected; a second shift register having output ports the number of which is smaller than the number of photodetection portions forming a vertical line out of the plurality of photodetection portions and smaller than the number of output ports of the first shift register, said second shift register being adapted for generating timing of movement of plural potential wells established in each of the vertical transfer lines; and a circuit for generating voltages of mutually different levels applied at predetermined timing to the transfer electrodes in the electrode string provided for each of the vertical transfer lines.
For providing an image-pickup apparatus without a need for providing storage areas for storing signal charges between the output ends of the vertical transfer lines and the horizontal transfer line, the image-pickup apparatus according to the present invention is driven so that in each of the vertical transfer lines the signal charge in the potential well, having been moved to the output end of the vertical transfer line, is transferred to the horizontal transfer line during a horizontal blanking interval. In this case, the image-pickup apparatus is provided with a first output electrode provided so as to cover the output ends of the respective vertical transfer lines adjacent to the horizontal transfer line and cover a part of the horizontal transfer line. Then the above vertical driving control system applies a predetermined voltage to the first output electrode so that the signal charges are transferred to the horizontal transfer line during a horizontal blanking interval and at timing when the potential well containing the signal charge reaches the output end of each vertical transfer line.
On the other hand, in order to realize an image-pickup pickup apparatus with high degrees of freedom of design as to the driving timing of the vertical transfer lines, the image-pickup apparatus according to the present invention preferably comprises a plurality of storage areas provided corresponding to the respective vertical transfer lines, each storage area being adapted for storing a signal charge, having been transferred in the each vertical transfer line, between the horizontal transfer line and the each vertical transfer line; a first electrode covering a part of the horizontal transfer line and a part of each of the plurality of storage areas; and a second output electrode covering a part of each of the plurality of storage areas and the output end of each of the vertical transfer lines. In this case, the above vertical driving control system applies a predetermined voltage to the first electrode at timing when a potential well containing a signal charge reaches the output end of each of the vertical transfer lines and also applies a predetermined voltage to the second output electrode during a horizontal blanking interval.
In this way, in each of the vertical transfer lines, a signal charge in a potential well, having been moved to the output end of each vertical transfer line, is temporarily withdrawn from the vertical transfer line, and after arrival of a next horizontal blanking interval, the signal charge temporarily withdrawn is outputted to the horizontal transfer line, which ensures degrees of freedom of design for the driving timing of vertical transfer lines.
Next, the image-pickup apparatus and the method for driving the apparatus according to the present invention can also prevent the negative effect due to the transfer leak of signal charge transferred in the vertical transfer lines. Namely, in each of the vertical transfer lines at a certain instant, a main potential well for transferring a signal charge and an auxiliary potential well for transferring a leak of the signal charge are established adjacent to each other along the vertical direction. Then the signal charge in the main potential well and the leak of the signal charge collected by the auxiliary potential well paired with the main potential well in each of the vertical transfer lines are outputted (or added) each onto the horizontal transfer line, thus realizing the prevention of the negative effect. The above pair of main potential well and auxiliary potential well are established by such arrangement that the above vertical driving control system simultaneously selects an electrode group for establishing at least the main potential well and an electrode group for establishing the auxiliary potential well from the electrode string provided for each of the vertical transfer lines. This structure can further increase the transfer efficiency of signal charge in the vertical transfer lines.
Further, for realizing an image-pickup apparatus having an electronic shutter function, the image-pickup apparatus according to the present invention is driven preferably so that in the above plurality of vertical transfer lines, after a lapse of a predetermined time from the time when undesired charges accumulated in the photodetection portions of a horizontal line selected out of the horizontal lines are transferred into the adjacent vertical transfer lines, signal charges accumulated in the photodetection portions are transferred to the adjacent vertical transfer lines and the undesired charges in the potential wells, having been moved to a predetermined portion of the vertical transfer lines, are drained away to the outside of the vertical transfer lines.
For realizing this driving method, the image-pickup apparatus according to the present invention comprises a plurality of discharge areas for sweeping an undesired charge having been transferred in each vertical transfer line away thereinto, each discharge area being a region provided between the output end of each of the vertical lines and the horizontal transfer line; and a third output electrode covering a part of each of the plurality of discharge areas and a part of each vertical transfer line corresponding thereto. Then the above vertical driving control system controls the associated gate structures so that after a lapse of a predetermined time from the time when the undesired charge accumulated in each photodetection portion of a horizontal line selected out of the horizontal lines is transferred to each vertical transfer line, the signal charge accumulated in the each photodetection portion is transferred to each vertical transfer line adjacent thereto, and in each vertical transfer line the vertical driving control system applies a predetermined voltage to the electrode group for establishing the potential well, thereby establishing the potential well to which the signal charge in the each photodetection portion is to be transferred. For transferring the undesired charge from each vertical transfer line to a corresponding one of discharge areas, this vertical driving control system applies a predetermined voltage to the third output electrode in each vertical transfer line, at the timing when the potential well containing the undesired charge reaches a portion adjacent to the corresponding discharge area.
When the above horizontal line is composed of a plurality of rows of photodetection portions adjacent to each other along the horizontal direction, it is also possible to realize an image-pickup apparatus having the interlace scan function. Specifically, the image-pickup apparatus according to the present invention is driven preferably so that a shift of a predetermined distance in the vertical direction is given to positions where potential wells for vertically transferring a signal charge accumulated in a photodetection portion forming the horizontal line are established successively in a first operation for reading an odd field with respect to those in a second operation for reading an even field. Also in this driving method the above vertical driving control system successively selects different electrode groups respectively in the first operation and in the second operation, out of the electrode string provided for each of the vertical transfer lines.
In addition, the image-pickup apparatus according to the present invention can also be driven so that in each of the vertical transfer lines, a plurality of horizontal lines are selected based on predetermined reading sections of horizontal lines and signal charges accumulated in the respective photodetection portions forming the respective horizontal lines spaced at a predetermined distance are transferred at once to the vertical transfer line. In this case, the basic structure is the same as the structure of the above-stated image-pickup apparatus, but the vertical driving control system in the image-pickup apparatus controls the gate structures corresponding to the respective photodetection portions forming the respective horizontal lines spaced at the predetermined distance and selected based on the predetermined reading sections of horizontal lines out of the plurality of horizontal lines each being one row or two or more rows of photodetection portions arranged along the horizontal line, and in each of the vertical transfer lines and in order to establish a plurality of potential wells having a length not smaller than a width of each horizontal line in the vertical direction and successively move the plurality of potential wells in the vertical direction, the vertical driving control system successively selects a plurality of electrode groups each including two or more transfer electrodes arranged consecutively from the above electrode string and successively applies a predetermined voltage to the plurality of electrode groups selected in each of the vertical transfer lines.
Further, the image-pickup apparatus comprises a pixel rearranging section for successively capturing outputs from the above horizontal transfer line, rearranging the outputs according to an original sequence of the horizontal lines, and thereby generating image signals of a frame unit or a field unit.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.